Poly(aryletherketone)s (PAEK) have been known for many years. Poly(etheretherketone) (PEEK) and poly(etherketone) (PEK) are the most common PAEK. PEK and PEEK are high-strength, radiation-resistant engineering plastics whose structures combine both ether and ketone groups. Both are thermally stable and highly resistant to chemicals.
PAEK can be prepared from a variety of starting materials, either via a nucleophilic route or an electrophilic route.
PEEK is the most commercially significant PAEK. There are many superlatives that can be used to describe the properties of PEEK, and it is regarded by many as one of the best performing thermoplastics.
PEEK has greater strength, heat resistance and rigidity than many of the other engineering thermoplastics, it has good mechanical properties, including impact resistance, low wear rate, excellent thermal oxidative stability, good dielectric properties, outstanding chemical resistance, good resistance to hydrolysis and a low coefficient of friction, but more importantly, these properties are also retained over a wide temperature range. PEEK has also an the material has one of the lowest smoke generation characteristics of the engineering thermoplastics. In addition, PEEK has good resistance to beta and X-rays, as well as exceptional resistance to gamma rays. These properties allow for ease of sterilization, and coupled with good biocompatibility, PEEK makes a strong candidate for medical applications.
PEEK can be prepared by various methods. One well known in the art method comprises reacting a substantially equimolar mixture of at least one bisphenol and at least one dihalobenzoid compound (for the two-monomer route) or at least one halophenol compound (for the one-monomer route) as described in Canadian Pat. No. 847,963. Preferred bisphenols in such a process are hydroquinone, 4,4′-dihydroxybiphenyl and 4,4′-dihydroxybenzophenone; preferred dihalobenzoid compounds in such a process are 4,4′-difluorobenzophenone, 4,4′-dichlorobenzophenone and 4-chloro-4′-fluorobenzophenone; preferred halophenols compounds in such a process are 4-(4-chlorobenzoyl)phenol and (4-fluorobenzoyl)phenol. Accordingly, PEEK homopolymers may notably be produced by the nucleophilic process as described in, for example, U.S. Pat. No. 4,176,222.
PEEK is generally prepared by reacting hydroquinone with 4,4′-difluorobenzophenone in diphenylsulfone in the presence of alkali-metal carbonates under an inert atmosphere at temperatures approaching the melting point of the polymer (>300° C.).
PEEK made via nucleophilic route is obtained as a white-off white powder. When crystallized from the melt, the product appears dark grey.
On the other hand, PEK has a higher Tg and a much higher degree of crystallinity than PEEK, hence higher temperature and chemical resistance. It also shows improved toughness.
PEK is generally produced by two nucleophilic routes: the two-monomer route or the one-monomer route. The two-monomer route, which is usually preferred, employs 4,4′-dihydroxybenzophenone in place of hydroquinone. The single-monomer route employs the alkali metal salt of 4-fluoro-4′-hydroxybenzophenone.
PAEK are known for their exceptional balance of technical properties, namely high melting point, good thermal stability, high stiffness and strength, good toughness and really excellent chemical resistance. Therefore, PAEK have potential for a wide variety of uses, and their favorable properties class them with the best of the engineering polymers. However, PAEK currently available to the trade suffer from certain disadvantages.
While prior art PAEK has acceptable melt stability, it may not be sufficient in certain demanding applications (such as melt filtration) where the PAEK are submitted to very high temperatures. PAEK featuring improved melt stability are thus needed by the art.
Attempts have already been made to improve the melt stability of PAEK.
U.S. Pat. No. 4,320,224 suggests to use a slight excess over equimolar of one of the dihalide, up to 5 mole %, results in the formation of halide end groups providing a polymer of greater thermal stability. However, due to the reversibility of the polymerization reaction, polymer with a high level of halide end groups is difficult to obtain (at the equilibrium there is still a substantial amount of OH end groups). This reference does not provide any quantitative information on the level of fluoride end groups required to achieve sufficient melt stability. Furthermore, precise control of the molecular weight using a stoichiometry imbalance requires a very high level of accuracy in the charging of the reactor, which is difficult to achieve at an industrial level. A process wherein the polymer is terminated when the target molecular weight has been reached is desirable.
Besides, U.S. Pat. No. 6,881,816 relates to the preparation of PEEK by an electrophilic route. It teaches that the nature of the end-group is critical for attaining PEEK featuring good thermal stability. PEEK prepared by the described process contains modified end-groups such as: -Ph, -Ph-CH3, -Ph-O—CH3, -Ph-O-Ph or -Ph-OH.
Prior art PAEK also suffer from the presence of defects. Defects are defined as local irregularities in an amorphous film, which lead to reduced transmission of light through the film. In a 50 to 51 pun-thick film illuminated with a halogen spotlight, any irregularity with a transmittance lower than 62% is a defect. Gels are defined as defects, with a transmittance of 30 to 62%, transmittance of above 62% being defined as transparent. In addition to an unacceptable cosmetic appearance, the presence of defects can affect the mechanical properties of the material in certain shapes with thin cross sections (such as films, fibers, wire coating).
Attempts have also already been made to reduce the defects as it may be observed on films made of PAEK.
U.S. Pat. No. 4,176,222, teaches that the combined use of sodium carbonate and potassium carbonate in the manufacture of poly(etheretherketone) avoid the presence of defects such as gels in compression molded film.
PAEK currently available to the trade have an inherent yellow to dark grey color as formed, which limits their use in certain specific applications where lighter colors are needed. PAEK having an improved, lighter color could find wider acceptance for many applications where color is a concern. There was also a long felt need to obtain low colored PAEK that remain also low colored after molding or melt processed. Although the effect on mechanical properties may be minimal, the cosmetic appearance of articles made from such polymers may be unacceptable in certain specific applications. Lower color PAEK are thus clearly needed by the art and would represent a significant improvement over the PAEK currently available to the trade.
Attempts have also been made in the past to improve the colour of PAEK.
DE 4121139 B4 describes the synthesis of PEEK and teaches that oxygen should be excluded from the reaction medium during the polycondensation reaction in order to obtain low colored PEEK. Highly condensed aromatic ring systems such as anthracene is also used in order to obtain low colored PEEK. However, condensed polyaromatics, such as anthracene and perylene, being highly toxic compounds, a complete removal of these compounds from the polymer is required for many sensitive applications (food contact, etc).
U.S. Pat. No. 6,881,816 deals also with the issue of the manufacture of low colored PEEK is also mentioned since it further discloses that the treatment of PEEK with organic solvents (and formic acid) improves the color of PEEK powder. However, these treatments are specific to PEEK made via an electrophilic route. Their role is to remove, with an organic basic solvent, strong acid residuals from the acidic solvents and to reduce, with formic acid, 9-phenylenexanthydrol end groups formed by intramolecular reaction on a ketone group catalyzed by a strong acid. Unfortunately, the above mentioned treatments do not improve the color of PEEK made via a nucleophilic route.
A process, described in EP 0211 693 and more precisely in its example 4, was also known in the art. This process does not lead to the obtention of high molecular weight polymers due to a serious stoichiometry imbalance of the monomers.
The problems as above detailed of providing a PAEK with superior melt stability, lower gel content and lower color can be solved by the present invention, as detailed below.
Surprisingly, the Applicant has found that the presence of fluorine end groups has a great impact, not only on the thermal stability of the PAEK, but also on the quantity of defects as measured on films and on their color.
It is an object of the present invention to provide PAEK which exhibit unexpectedly advantageous properties.
It is also an object of the present invention to provide an improved process for the manufacture of PAEK of superior quality allowing an easy molecular weight control, a controlled microstructure of the polymer, and a control on the chain ends termination.
Besides, it is also an object of the present invention to provide PAEK which exhibit superior melt stability, less defects on films as well as a lower color, compared to the prior art PAEK.